In the established art of measuring selected radio frequency (RF) signals a broad spectrum of techniques and measurement apparatus is available. In constantly striving to improve system components, reduction in self-generated noise, and rejection or suppression of transient or other inherent noises are problems which are always involved. To reduce noise, such as that inherent in an oscillator or amplifier so that low level intelligent signals can be measured, the noise level and characteristics must be identified. Thus in apparatus for providing measurement of noise in the frequency domain, the noise becomes the signal of interest.
The use of a frequency discriminator to measure near carrier frequency modulation noise of a microwave signal was described by Ashley et al in the IEEE Transactions on Microwave Theory and Techniques, Volumn MTT-16, No. 9, September 1968, pages 753-760. Prior art discrimination at microwave frequencies, as described in this paper depended on a microwave cavity as a resonant circuit. Prior art discrimination at very high frequencies (VHF) utilize such components as slope detectors and ratio detectors, depending on the lumped circuit elements of inductance and capacitance, to form resonant circuits. At ultra high frequencies (UHF) between the VHF and microwave regions, there is little prior art on such discrimination means. Most measurements in this area are made by beating or heterodyning the UHF signal with a local oscillator to obtain a VHF signal which is analyzed with a discriminator at the VHF frequency range. Accounting for the FM noise of the local oscillator is a well established problem in these areas. Both of these techniques provide a means for rejecting residual amplitude modulation (AM) noise on the signal under test; for example, the VHF discriminators usually employ a limiter or ratio detector. The microwave cavity discriminator rejects AM noise, allows the input signal level to be increased, and provides the greatest discriminated output to improve the signal-to-threshold ratio, with the threshold being the lowest value of noise established or obtained from within the analyzer system.
Among the discriminators discussed in prior art literature which might be useful in obtaining FM noise measurements at UHF or lower microwave frequencies is the transmission line discriminator. An article entitled "Stability Measurement Techniques in the Frequency Domain" by R. A. Campbell, IEEE-NASA Symposium on Short Term Frequency Stability, NASA SP-80, Nov. 23-24, 1964, pages 231-235, discloses such discriminators. Various aspects of detection equipment and calibration are disclosed. Also an article entitled "Single Hybrid Tee Frequency Discriminator" is disclosed by J. Nigrin et al in the IEEE Transactions on Microwave Theory and Techniques, Volumn MTT-23, No. 9, September 1975, pages 776-778. The discriminator is tuned by a movable short and has properties comparable to those of a phase discriminator. Additional prior art includes U.S. Pat. No. 3,675,124 entitled "Apparatus for Measuring Frequency Modulation Noise Signals and for Calibrating Same" by J. R. Ashley et al. This apparatus employs an auxiliary injection phaselocked oscillator driven by a test oscillator and relies on a discriminator cavity resonator which must be accurately tuned to the exact operating frequency for proper operation. Ashley et al discusses in column 6 the difficulty of making signal measurements where limited power outputs make the use of prior art methods ineffective.